1. Field of the Invention
This invention relates to a process of making a self-emulsifiable polyolefin aqueous emulsion of relatively high molecular weight and to a hybrid polyolefin-polyurethane dispersion obtained therefrom.
2. Description of Related Art
The use of polyolefins such as polyethylene, polypropylene, copolymers of ethylene and alpha-olefins, and ethylene-propylene-diene terpolymer (EPDM) continues to grow at a rate faster than that of many other resins due to their low cost and excellent balance between design flexibility, mechanical properties and recyclability.
In order to improve the mechanical properties of the myriad of consumer goods made from the foregoing types of polyolefins, these resins are commonly reinforced with fillers, e.g., glass fibers. Glass fiber-reinforced polyolefins have better dimensional stability, tensile strength, flexural modulus, flexural strength, impact resistance and creep resistance than non-reinforced polyolefins. The glass fibers are generally treated with a sizing composition that performs several functions. It protects the fibers in a strand during processing and it has the capability to couple or adhere the glass fibers to the polyolefin. Aqueous polyolefin emulsions have been found to be useful for glass fiber treatment in thermoplastic polyolefin (TPO) reinforcement. However, they are not good film-formers and require the use of another film-forming component to achieve additional useful properties such as good strand integrity.
Due to the generally poor photo stability of polyolefins, parts made from, e.g., TPO are not used for the exterior parts of automobiles without first being given a protective coating. Another reason for using coatings is the desire that parts made from these resins match the color and gloss of the main automobile body. However, most coating systems cannot be used on polypropylene and other TPO substrates due to poor adhesion. This lack of adhesion can be attributed to the generally poor wettability, good solvent resistance and very low surface energy of the TPO. TPO belongs to the group of polymers with the lowest surface energy, next to that of the polysiloxanes and fluoropolymers.
The most common approach to overcoming the technical difficulties encountered in the coating of TPO and similar polyolefins in general is to use an adhesion-promoting primer coating based on chlorinated polyolefin resin. Primers often contain significant amounts of volatile organic compounds (VOC). This is a significant disadvantage for a finisher who is required to comply with strict federal and state environmental emission standards. In addition, halogen-containing organics such as chlorinated polyolefin present environmental concerns of their own.
Therefore, it would be advantageous to replace a chlorinated polyolefin based primer with one having good adhesion to TPO but lacking a significant VOC component and in addition, lacking a halogen-containing organic component.
As stated above, high molecular weight polyolefins have been shown to have improved physical properties in sizing and in promoting adhesion. But high molecular weight polyolefins such as isotactic polypropylene are known for having high melting temperatures and high melt viscosities. This presents a dual problem. First of all, the usefulness of a polyolefin emulsion is generally determined by the compatibility of the emulsified material with the polyolefin. The increased melt viscosity of a high molecular weight polyolefin decreases its compatibility in an emulsion. This is generally dealt with by combining the high molecular weight polyolefin with a low molecular weight polyolefin in a sealed pressure vessel to a temperature above the melting point of the high molecular weight polyolefin. This combination of a low molecular weight polyolefin to the high molecular weight polyolefin reduces the melt viscosity of the overall mixture. The resultant polyolefin must then be combined with water at the elevated temperature and pressure to form a stable aqueous emulsion.
Secondly, methods for emulsification of functionalized polyolefins, known in the art, require mixing of the molten polyolefin with water in the presence of colloidal stabilizers. This poses a problem of the cost of the equipment and the extra time required in using a pressure vessel.
Therefore, it would be advantageous to develop a method for emulsification of functionalized polyolefins at normal atmospheric pressure. These polyolefins could then be emulsified at any time and at any pressure.
Various methods have been described in the prior art for the formation of high molecular weight polyolefin emulsions.
U.S. Pat. No. 4,283,322 describes the use of low molecular weight amorphous carboxylated polypropylene along with isotactic carboxylated polypropylene for making stable emulsions. However, this patent does not address the difficulty of making emulsions of high molecular weight isotactic polypropylene.
U.S. Pat. No. 5,389,440 describes a method for the emulsification of polyolefins having molecular weight greater than 10,000 where a fatty acid material is used to successfully emulsify a high molecular weight polyolefin. This process produces emulsions with relatively large particles, which tend to separate upon standing (creaming).
U.S. Pat. No. 6,166,118 describes a process for emulsification of high molecular weight functionalized polyolefins, which employs xe2x80x9can indirect, or dilution method of pressure emulsificationxe2x80x9d. Although it still requires the preparation of a xe2x80x9cpre-emulsionxe2x80x9d, or xe2x80x9cemulsion concentratexe2x80x9d, with its subsequent dilution to practically usable concentration.
French Patent No. 2,588,263 described a technique, in which high molecular weight crystalline polyolefin is dissolved in hydrocarbon solvent first, to make it easier to disperse. This technique requires subsequent removal of the solvent and brings safety concerns.
It is an object of the present invention to provide a process for making a polyolefin of relatively high weight average molecular weight, i.e., of at least about 30,000 that is self-emulsifiable under normal atmospheric conditions and to further make an aqueous polyolefin emulsion thereof.
It is a further object of the present invention to provide a hybrid polyolefin-polyurethane dispersion based on the foregoing emulsion of polyolefin, advantageously, one which lacks any appreciable quantity of volatile organic compound and/or halogen-containing organic compound.
In keeping with these and other objects of the invention there is provided a process for preparing an aqueous emulsion of self-emulsifiable polyolefin comprising the steps of:
(a) providing a mixture of:
1. at least one first polyolefin possessing a first reactive functionality and having a weight average molecular weight of at least about 30,000; and
2. at least one hydrophilic polymer possessing a second reactive functionality being reactive with the first reactive functionality of polyolefin (1) and having a weight average molecular weight less than that of polyolefin (1); and
(b) heating the mixture of step (a) to a temperature at or above the melting point of polyolefin (1), first reactive functionality of polyolefin (1) reacting with second reactive functionality of hydrophilic polymer (2) at said temperature to provide a self-emulsifiable polyolefin; and
(c) adding an emulsion-forming amount of water to the self-emulsifiable polyolefin of step (b) to provide an aqueous emulsion of the self-emulsifiable polyolefin.
In addition, there is provided a process for making a hybrid aqueous dispersion of polyolefin and polyurethane which comprises:
a) providing at least one water-dispersible polyurethane prepolymer;
b) dispersing the water-dispersible polyurethane prepolymer of step (a) in at least one aqueous emulsion of polyolefin prepared by the foregoing emulsification process; and,
c) reacting dispersed polyurethane prepolymer with at least one difunctional chain extender to provide a hybrid aqueous dispersion of polyolefin and polyurethane.
The hybrid aqueous dispersion of polyolefin and polyurethane possesses good film-forming properties at room temperature and is useful in coatings processes where it improves the adhesion of coatings to various kinds of substrates. Another application of the foregoing hybrid aqueous polyolefin-polyurethane dispersion is for the treatment of glass fibers used as fillers in the reinforcement of polyolefin resins such as PE, PP, ethylene/alpha-olefin copolymer, TPO, and the like. In a preferred embodiment, the hybrid aqueous polyolefin-polyurethane dispersion contains no appreciable quantity of volatile organic compound(s) and/or chlorine-containing organic compound(s).